This invention is directed to electrical braking of direct current electric motors and, more particularly, to apparatus for smoothly transitioning into and out of regenerative electrical braking.
Electric vehicles such as locomotives, transit cars, forklift trucks or on-road vehicles which are powered by electric traction motors generally depend upon electrical braking by the traction motors to assist mechanical or friction brakes in stopping the vehicles. In order to provide this electrical braking effort, the traction motors are electrically controlled to operate as electrical generators driven by a rolling wheel of the vehicle. In operating as generators, the traction motors are effective to convert the kinetic energy of the vehicle to electrical energy. The chosen method of disposing of this electrical energy classifies the type of electrical braking being utilized. In general, only two types of electrical braking are in common use: dynamic braking in which the electrical energy is converted to thermal energy in resistive loads; and regenerative braking in which the electrical energy is transferred back to the power source. A subset of dynamic braking is "plug" braking in which case the resistance of the armature of the electric motor itself is utilized as the resistive load.
It is obvious that regenerative braking is a preferred method to use if the power source is capable of accepting the regenerative energy and using it for other loads or storing it for later use. However, efficient use of regenerative braking requires that the current developed by the electric motor be of greater magnitude than the field excitation necessary to generate the current. As is well known, the current developed by the electric motor is a function of motor field current and armature rotational velocity. Accordingly, as vehicle speed is reduced, the ability of the electric motor to regenerate energy is also reduced. In those systems in which a series wound traction motor is used such that a portion of the armature regenerated current is used as field current for the electric motor, a point will be reached at some speed at which all of the regenerated energy is necessary to maintain field excitation at a level to produce the desired braking torque and no energy is available to be regenerated to the source. At this speed, regenerative braking must be terminated and some form of dynamic or plug braking initiated. Thus, two separate transitions must be undertaken when the electric vehicle transitions from a propulsion to a braking mode of operation: a first in going from propulsion to regenerative braking followed by a second in going from regenerative to dynamic braking.
When a series wound electric traction motor is used for a propulsion vehicle, the transition from a propulsion mode to an electrical braking mode requires that either the field winding or the armature of the motor be reconnected in a reverse configuration from that used in the propulsion mode. In most instances, the preferred method is to reverse the field winding connections. Since this reversal of the field winding connections will also reverse the polarity of the voltage on the armature which will now act as a generator, a contactor is normally used to reconnect the armature into the proper arrangement for regenerative braking. In many instances the residual flux in the armature and field winding will be sufficient to permit regenerative braking to be established as soon as the reconnection has occurred. However, in some instances the residual flux is either of the wrong polarity or is insufficient to permit regenerative braking to be established. In order to assure that regenerative braking is established, many prior art systems have incorporated a resistance which connects the motor armature to the battery in such a manner that a current path through the motor is established in a direction to insure initiation of regenerative braking. However, such a resistance, regardless of size, may consume from five to ten percent of the available regenerative energy and in addition create excessive heat dissipation problems. For example, a five thousand pound vehicle traveling at ten miles per hour has an initial inertial energy of twenty-two thousand seven hundred twenty-three watt-seconds.
When the speed of the vehicle has dropped to such a level that the motor velocity is insufficient to sustain the regenerative current at the desired braking torque, and therefore to regenerate energy into the power source, the control circuitry must convert the motor power circuit from a regenerative configuration to a dynamic brake configuration. In performing the switching function, field current is normally reduced to zero to reduce the armature potential to zero whereby the armature can again be connected to the power source in its driving or propulsion mode configuration without producing large current transients. With no armature current and no armature potential, a contractor can be used to reconnect the armature to the power source without arcing or burning the contactor tips. After reconnection into a dynamic braking mode, field current again builds up to permit dynamic or plug braking of the electric motor. The transition from regenerative braking to dynamic braking with its attendant loss of braking effort due to reduction of field current and armature potential permits braking torque to die out and the gearing between the electric motor and the drive wheels to relax. The subsequent re-initiation of braking torque will result in a jerk reaction of the vehicle and, in the case of a forklift truck, could result in dropping of objects being carried.
It is an object of the present invention to provide an improved electrical braking arrangement for an electrically propelled vehicle.
It is a still further object of the present invention to provide an electric vehicle with a control circuit to allow smooth transitioning between propulsion and regenerative braking.
It is still another object of the present invention to provide an improved braking mode control system which allows smooth blending between regenerative and dynamic braking of an electric vehicle.
In accordance with the present invention, a series connected electric traction motor is arranged to provide propulsion power to the wheels of an electric vehicle. The control system for the traction motor includes apparatus for reversing the relative polarity of the field winding and armature winding in order to enable transition from a propulsion to a braking mode and also to permit reverse propulsion of the motor. Apparatus is provided for establishing a reverse current path between the motor armature and a DC power source such that the motor armature may regenerate electrical energy into the power source during braking. Controllable resistance means is connected between the armature and the power source to provide an initial current path through the armature to establish the proper polarity of motor flux to enable regenerative braking. The resistance means includes apparatus for removing it from the circuit when regenerative braking has been initiated. An additional resistive brake blending device is provided in the system for permitting braking current to continue to flow when the system is switched between regenerative braking and dynamic or plug braking.